1. Field of the Invention
The process of the present invention relates to the improvement of the properties of coal, and is classified generally in the class relating to fuels and igniting devices.
2. The Prior Art
With the present world-wide emphasis on the energy crisis and the rapidly diminishing sources of oil, increased attention by both government and private organizations is being given to coal as a source of energy, especially for the generation of electricity. This country has vast resources of coal for development as other sources of energy diminish.
Depending upon their origin, coals contain varying amounts of iron disulfide (iron disulfide is hereinafter referred to as pyrite whether crystallized as pyrite or marcasite) from which sulfur dioxide is formed as a combustion product when coal is burned. This is a tremendous disadvantage to the use of coal as an energy source, particularly in view of the current emphasis on pollution controls as illustrated by present federal emission control standards for sulfur dioxide. Illustrating the enormity of the sulfur dioxide emission problem is the fact that large transportation expenses are incurred by coal users in transporting Western and European coal of relatively low sulfur content long distances to supplant available high sulfur-containing coals in order to comply with sulfur dioxide emission standards. At this time there are no effective means available which are commercially feasible for absorbing the large amounts of sulfur dioxide emitted by the combustion of coal to produce heat and electricity. One solution to the problem is to separate the sulfur-bearing pyrite from the coal before it is burned.
Coals also contain, depending upon their origin, various amounts and kinds of minerals which form ash when the coal is burned. The ash also is a disadvantage to the use of coal as an energy source, since it contributes no energy value during combustion. The ash causes a dilution of the calorific value of the coal, and causes a waste disposal problem and a potential air pollution problem.
The problem of separating pyrite and/or other impurities from raw coal is not new and a number of methods have been extensively tested over the years. Among these are methods which employ the difference in specific gravity between coal particles and the impurity particles or differences in their surface, electrostatic, chemical, or magnetic properties. For various reasons difficulties are encountered in making an efficient separation of pyrite or other impurities from coal which has been ground fine enough to substantially liberate impurity particles from coal particles. In water systems this difficulty is related to the slow settling rate of fine particles, and in air systems to the large difference in specific gravity between air and the particles. However, for magnetic separations the magnetic attraction force acting on small magnetic particles is many times greater than the opposing force, which is usually a hydraulic drag and/or gravity force.
For the separation of pyrite or other impurities from raw coal the success of a magnetic process is dependent upon some effective treatment process for selectively enhancing the magnetic susceptibility of the pyrite or other impurity. Coal particles alone are slightly diamagnetic while pyrite and many other mineral impurities are weakly paramagnetic; however, their paramagnetism has not been sufficient to economically effect a separation from coal. However, effective beneficiation of coals can be made if the magnetic susceptibility of pyrite or other impurities is increased. For pyrite it has been estimated that a sufficient increase in susceptibility can be achieved by converting less than 0.1 percent of pyrite in pyritic coal into ferromagnetic compounds of iron ("Magnetic Separation of Pyrite from Coals," Bureau of Mines Report of Investigations 7181, p.1).
In discussing the use of heat to enhance the paramagnetism of pyrite it is stated in the above report (p.1) that ferromagnetic compounds of iron are not formed in significant quantities at temperatures below 400.degree. C., and that such conversion occurs in sufficient quantities to effect beneficiation only at temperatures greater than 500.degree. C. As this is above the decomposition temperature of coal, the use of heat to enhance the magnetic susceptibility of impurities does not appear feasible. Further, other methods for enhancing the paramagnetism of pyrite to permit its separation from coal have not been encouraging.
U.S. Pat. No. 3,938,966 discloses a process for improving coal wherein the raw coal is reacted with substantially undecomposed iron carbonyl which alters the magnetic susceptibility of certain impurity components contained in the raw coal, thereby permitting their removal by low-intensity magnetic separators. This process represents a noteworthy advance in the art, as treating coal in accordance with this process may substantially remove impurities such as pyrite, a primary contributor to sulfur dioxide pollution problems. The process of this patent, however, does not appear to possess universal applicability with an equal degree of success in that while many coals are substantially enhanced by this treatment, certain other coals are not as receptive. It has been discovered by the inventors of the present application that pretreating coal to remove at least a portion of elemental sulfur present under various conditions as hereinafter presented substantially enhances the effectiveness of the process of this patent. The process of the present invention therefore constitutes, in part, an improvement of the process described in U.S. Pat. No. 3,938,966, in accordance with the discussion presented hereinafter.